1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system, such as a laser printer, a copier, and a facsimile.
2. Description of the Related Art
Conventionally, as an image forming apparatus of an electrophotographic system, there is an image forming apparatus of an intermediate transfer system that primarily transfers a toner image, which is formed on an electrophotographic photosensitive member (photosensitive member) as an image bearing member, onto an intermediate transfer member, and then, secondarily transfers the toner image onto a recording material. Further, as the image forming apparatus of the intermediate transfer system, for example, a tandem-type (or in-line type) image forming apparatus is known, which primarily transfers toner images of a plurality of colors respectively formed on a plurality of photosensitive members onto an intermediate transfer member so that the toner images are superimposed successively, and then, secondarily transfers the toner images onto a recording material at a time. In the image forming apparatus of the intermediate transfer system, toner (residual toner) remains on the intermediate transfer member after the secondary transfer step. Therefore, the image forming apparatus of the intermediate transfer system is provided with an intermediate transfer member cleaning device for removing and collecting the residual toner.
Japanese Patent No. 3267507 discloses an intermediate transfer member cleaning device that allows residual toner to be charged oppositely to a normal charge polarity of toner, and then, transfers the residual toner from an intermediate transfer member onto a photosensitive member in a primary transfer part of an image forming part immediately. Then, the intermediate transfer member cleaning device allows the residual toner to be collected by a cleaning device of a photosensitive member. According to this method, a waste toner container for collecting toner dedicated for an intermediate transfer member can be eliminated. Further, the intermediate transfer member can be cleaned simultaneously with the primary transfer. Further, according to this method, there is an advantage in that a dedicated toner containing mechanism for collecting the residual toner is not required. In the case of adopting the method of collecting residual toner transferred from the intermediate transfer member onto the photosensitive member as described above, the intermediate transfer member cleaning device including a toner charging roller as a toner charging member that charges residual toner is provided. The residual toner adhering to the toner charging roller during an image formation operation is discharged (that is, transferred) onto the intermediate transfer member at a predetermined timing during an operation after the image formation operation. As a specific method of discharging the residual toner from the toner charging roller, a method of repeating application of a negative voltage and application of a positive voltage with respect to the toner charging roller alternately, with a time period of a substantially one round of the toner charging roller being a half period may be conceived.
By the way, in order to reduce a size and cost of an image forming apparatus, it is effective to share a high-voltage power supply circuit used in the image forming apparatus.
However, when the high-voltage power supply circuit is shared, there is such a risk that toner may not be collected exactly from an intermediate transfer member in some cases. For example, when a power source for discharging toner from a toner charging roller onto an intermediate transfer member and a power source for collecting toner, which is discharged onto the intermediate transfer member, in a photosensitive member are provided in common, the following problem arises. That is, an application timing of a voltage for discharging toner is not matched with an application timing of a voltage for collecting toner which has been discharged (discharged toner), and the discharged toner cannot be collected in the primary transfer part to remain on the intermediate transfer member.